In the prior art recycling is attained by first mincing the articles made of materials that are different from one another, with the aim of obtaining a granular material having an average size in the order of 0.5-25 mm.
The granular material comprises a mass made up of granules of various materials which in order to be re-utilised have to be separated such as to obtain homogeneous masses comprising granules of the same material.
Granule separation techniques are known for plastic materials which use the triboelectric effect, which is based on the following phenomenon.
By causing particles of a first material A to rub against particles of a second material B, particles A and B become electrostatically oppositely charged.
By the same process of rubbing, particles of a same type do not become charged at all.
The prior art describes various triboelectric separation methods of particles of different materials, which methods comprise:    a. undifferentiated collection of objects made of a plastic material;    b. cleaning of the objects;    c. mincing the objects in order to reduce them to particles of a homogeneous size;    d. an electrostatic charging operation;    e. transit of the electrically-charged particles in an electrical field.
U.S. Pat. No. 6,903,294 describes a separation device comprising a section for electrostatically charging by reciprocal rubbing of different types of synthetic material, a first electrostatic separation station located superiorly, at least a second electrostatic separation station located at a lower level, and a collection station for the separated particles, where the stations are provided with a rotary-drum metal electrode having a part on which the particles are made to fall, and an adjacent electrode charged oppositely in order to create an electrical field between the first and the second electrode, such that the particles falling on the first electrode and passing through the electrical field separate electrostatically according to their polarity and their charge.
U.S. Pat. No. 6,927,354 comprises a device for electrically charging particles of an electrically-insulating material comprising a metal cylinder internally of which the particles are made to transit between an inlet mouth and an outlet mouth. The cylinder is set in rotation about the axis thereof which is orientated such that the particles rub against one another and against the wall of the cylinder, thus taking on an electrostatic charge.
An electrostatic separation turret downstream of the cylinder separates the particles according to their charge.
U.S. Pat. No. 5,289,922 illustrates the electrostatic separation of a mixture of plastic materials by passing the granules of material in a rotary cylinder which can be made of metal or constituted by one of the materials to be separated, and precisely by the material that is present in the lowest quantity in the granular mass.
When the cylinder is constituted by the smallest material in the granular mass, it has the task of increasing the electrostatic charge of the granules of material present in the greatest quantity in the granular mass.
The electrostatic charging, in the prior art, is created by advancing the mass of the particles on a surface, so that when in contact with one another they are obliged in some way to rub against one another, losing or receiving electrons according to their nature.
The prior art suffers from a certain number of drawbacks, among which of not least importance is the fact that generally the reciprocal dragging between particles is not sufficient nor sufficiently homogeneous to give the particles an advantageous quantity of charge.
U.S. Pat. No. 6,681,938 discloses an improved or enhanced triboelectrostatic separator which provides for multiple means of egress for components from the separation of fly ash.
Sized particles make contact with a charging surface to impart positive, negative or no charges on the particles. Some of the particles, depending from their chemical composition, become positively charged, other particles become negatively charged and some particles are not charged at all.
The charged and uncharged particles are passed through an electrostatic separator consisting of a plurality of conducting electrodes or charged louvered plates, across which a high voltage is applied.
The known systems do not, in summary, enable control to be made of the charge density taken on by the various particles, but only the polarity of the charge.
As the force to which the particles are to be subjected when they cross the electrical field depends on the density of the load, which in turn depends on the mass of particles subjected to rubbing, sometimes it is not possible to separate the two different materials from one another which charge up with the same polarity and are present in such proportions as to absorb the same charge density.